Charles Onyutha

Empirical statistical characterization and regionalization of amplitude–duration–frequency curves for extreme peak flows in the Lake Victoria Basin, East Africa

Abstract This paper focuses on a regionalization attempt to partly solve data limitation problems in statistical analysis of high flows to derive discharge–duration–frequency (QDF) relationships. The analysis is based on 24 selected catchments in the Lake Victoria Basin (LVB) in East Africa. Characteristics of the theoretical QDF relationships were parameterized to capture their slopes of extreme value distributions (evd), tail behaviour and scaling measures. To enable QDF estimates to be obtained for ungauged catchments, interdependence relationships between the QDF parameters were identified, and regional regression models were developed to explain the regional difference in these parameters from physiographic characteristics. In validation of the regression models, from the lowest (5 years) to the highest (25 years) return periods considered, the percentage bias in the QDF estimates ranged from –2% for the 5-year return period to 27% for 25-year return period. Editor D. Koutsoyiannis

Temporal and spatial variability of extreme river flow quantiles in the Upper Vistula River basin, Poland

Temporal and spatial variability in extreme quantile anomalies of seasonal and annual maximum river flows was studied for 41 gauging stations at rivers in the Upper Vistula River basin, Poland. Using the quantile perturbation method, the temporal variability in anomalies was analysed. Interdecadal oscillating components were extracted from the series of anomalies using the Hilbert-Huang transform method. Period length, part of variance of each component, and part of unexplained variance were assessed. Results show an oscillating pattern in the temporal occurrence of extreme flow quantiles with clusters of high values in the 1960–1970s and since the late 1990s and of low values in the 1980s and at the beginning of the 1990s. The anomalies show a high variability on the right bank of the Upper Vistula River basin during the summer season with the highest values in catchments located in the western and south-western parts of the basin. River flow extreme quantiles were found to be associated with large-scale climatic variables from the regions of the North Atlantic Ocean, Scandinavia, Eastern Europe, Asia, and, to a lesser extent, the Pacific Ocean. Similarities between temporal variability of river flows and climatic factors were revealed. Results of the study are important for flood frequency analysis because a long observation period is necessary to capture clusters of high and low river flows.

Temporal and spatial variability of flood quantiles in the Upper Vistula River basin, Poland

Temporal and spatial variability in extreme quantile anomalies of seasonal and annual maximum river flows was studied for 41 gauging stations at rivers in the Upper Vistula River basin, Poland. Using the quantile perturbation method, the temporal variability in anomalies was analysed. Interdecadal oscillating components were extracted from the series of anomalies using the Hilbert-Huang transform method. Period length, part of variance of each component, and part of unexplained variance were assessed. Results show an oscillating pattern in the temporal occurrence of extreme flow quantiles with clusters of high values in the 1960–1970s and since the late 1990s and of low values in the 1980s and at the beginning of the 1990s. The anomalies show a high variability on the right bank of the Upper Vistula River basin during the summer season with the highest values in catchments located in the western and south-western parts of the basin. River flow extreme quantiles were found to be associated with large-scale climatic variables from the regions of the North Atlantic Ocean, Scandinavia, Eastern Europe, Asia, and, to a lesser extent, the Pacific Ocean. Similarities between temporal variability of river flows and climatic factors were revealed. Results of the study are important for flood frequency analysis because a long observation period is necessary to capture clusters of high and low river flows.

Decadal Analysis of River Flow Extremes Using Quantile-Based Approaches

Next to the traditional analysis of trends in time series of hydro-climatological variables, analysis of decadal oscillations in these variables is of particular importance for the risk assessment of hydro-climatological disasters and risk-based decision-making. Conventional parametric and nonparametric tests, however, need implementing a set of background assumptions related to serial structure and statistical distribution of data. They neither focus on the extreme events and their probability of occurrence. In order to get rid of these limitations, we suggest a modified version of the Sen Method (SM), combined with the Quantile Perturbation Method (QPM) for examining temporal variation of extreme hydrological events. The developed method is tested for decadal analysis of monthly and annual river flows at 10 hydrometric stations in the Qazvin plain in Iran. The results show oscillatory patterns in extreme river flow quantiles, with a positive anomaly during the 1990s and a negative one during the 2000s. It is also shown that the concurrent use of the two methods allows to set a complete picture on the temporal changes in high and low extremes in historical river flow observations in different seasons.

How well do climate models reproduce variability in observed rainfall? A case study of the Lake Victoria basin considering CMIP3, CMIP5 and CORDEX simulations

In this study, how well the climate models reproduce variability in observed rainfall was assessed based on General Circulation Models (GCMs) from phase 3 and phase 5 of the Coupled Model Inter-comparison Project, i.e., CMIP3 and CMIP5, respectively as well as the Regional Climate Models (RCMs) of COordinated Regional Climate Downscaling EXperiment (CORDEX) over Africa. Observed and climate model based daily rainfall across the Lake Victoria Basin, which is one of the wettest parts of Africa, was considered. Temporal variability was assessed based on the coefficient of variation of daily and annual rainfall, and the maximum dry and wet spell in each year. Furthermore, variation in daily rainfall was assessed in terms of the long-range dependence. Comparison of variability results from observed and climate model based rainfall was made. It was found that the capacity to reproduce variability in observed wet and dry conditions depends on the specific GCM (of CMIP3 or CMIP5) or CORDEX RCM. However, the CORDEX RCMs replicated variability in observed daily rainfall better than the CMIP3 and CMIP5 GCMs. This was due to the spatial resolutions of the CORDEX RCMs which are higher than those of the CMIP3 and CMIP5 GCMs. The ensemble mean of the coefficients of correlation between the variability in observed and that of climate model based rainfall was close to zero for both the GCMs or RCMs. This suggests that analyses can be done on a case by case basis. In other words, GCMs or RCMs which adequately reproduce variability in observed wet and dry conditions can be considered for further statistical analysis of the changes especially on the basis of statistical methods for downscaling. For daily timescale, both the GCMs and RCMs from all the three sets of climate models generally exhibited poor performance in capturing the time of occurrences and the magnitudes of rainfall events (when considered in a combined way). To reliably assess long-term rainfall changes, it is vital to characterize natural variation in terms of the statistical dependence. With respect to natural variability of rainfall at local scales, there is room for further improvement of the climate models; however, whether theory of fractals and/or concepts of scaling behavior or self-similarity can explicitly contribute in that respect is a crucial consideration. Results from this study gave some insights in the reasonableness of the future rainfall projections.

Uncertainties in Impact Assessment of Climate Change on Rainfall Extremes in the Lake Victoria Basin

T are approximately 82 million ha of timberland and more than 13 million ha of southern pine plantations in the southeastern United States. In this region, slash pine (Pinus elliottii Engelm. var. elliottii) and loblolly pine (Pinus taeda L.) are the most important species, accounting for more than 90% of the planted seedlings in the US. The physiological-process based model, 3-PG (physiological process predicting growth), has been widely applied to estimate the effects of management, climate and site characteristics on different stand level attributes such as stem volume growth, biomass dynamics or water use efficiency. This model uses species-specific physiological traits in conjunction with empirical treeand stand-level dynamics attributes to quantify stand growth and dynamics. Recently, the model was parameterized for loblolly and slash pine stands. The model was validated against a large number studies and operational plots across the natural range of distribution of both species. In this study, we used the model to estimate the impact of future climate change scenarios (using RCP 4.5 and 8.5) on stand dynamics and productivity in SE United States.T occurrence of dry & wet events causes major adverse impacts in the Canadian Prairies. In the past, dry events, which are typically agricultural and/or hydrological droughts, have been common in the region. At least five major drought episodes have occurredin the Canadian Prairies during the past 120 years. These include multi-year droughts in the 1890s, 1910s, 1930s, late 1950s to early 1960s, 1980s, 1999-2005, 2009-10. The years 2001-2004 resulted in one of the largest area multi-year drought. Almost half of the agricultural prairies were in severe drought or worse during 2001. For the older droughts of the 1900 to 1950 period, the 1930s were the largest multi-year droughts, and 1961 and 1919 were the single-year severe droughts that covered the largest area. According to Bonsal et al. (2011), some of the major droughts have migrated into the Canadian Prairies from the United States Great Plains, including the 1999-2005 drought. This means that it is important to monitor the northern US droughts for expansion or migration into Canada.In the future, the frequency of both droughts and intense precipitation areexpected to increase. In fact, the review by Wheaton et al. (2013) found that (i) multi-year droughts (e.g., 5 to 10 years, or so) would occur more than twice as often forthe period to 2100; (ii) they would come with more evaporative power and be more intense as they will have much higher temperatures and much longer warm seasons; and (iii) they would cover much more area than even the across-Canada drought of 2001-2002. Droughts of such magnitudes would have devastating impacts on the Prairies, economically, environmentally, and socially, particularly on those industries where weather is a major determining factor to their survival and performance. One such industry is agriculture. It is the contention of this paper that estimation of impacts of the past droughts has been limited in scope, and may have underestimated these impacts. This under-estimation may be a result of several reasons: (i) Past studies have typically measured these impacts for only the drought period. In reality a drought would affect some of the enterprises (such as livestock production) that take several years to completely result in an economic gain to the producers. (ii) Although private costs of the people impacted by droughts are measured, the macro-level changes causing hardships to people and businesses are not measured. (iii) Droughts may impact ecological goods and services which can then affect the society at large. (iv) Loss to human capital through stress, sickness, and loss of employment as well as other changes that lead to lost productivity are typically not included. (v) Fiscal costs to the governments and their resulting impact on other programs have not been estimated. (vi) Economic hardships to smaller communities and people living therein have also not been addressed. We recommend that future investigations attempt to redress these deficiencies in the method of estimation of social, environmental and economic impacts of droughts. Impacts of the recent severe droughts in the US Plains and Canadian Prairies are reminders that current adaptation can be improved.T like other countries is impacted by climate change and climate variability. Floods and drought are the two major extreme climate events in Tanzania. In recent years (2009-2011), heavy rains accompanied with strong winds have left thousands of people displaced and without food. Heavy rains and floods have resulted into loss of life, livestock and crops; an increase in vector and water-borne diseases; food shortages, internal displacement, and increased disease transmissions, damage to properties, destruction of the environment and economy. On the other hand, six droughts have occurred between 1980-2008 where 7.96 million people were affected, resulting into famine, loss of life, crop failure, lower water availability and quality and electricity rationing. In the Kilimanjaro region, reductions of ice cap and reported glacier vertical retreat have been observed on Mount Kilimanjaro. During the past three to five decades, there has been a steady increase in temperature with decreasing water levels in Lakes in various parts of the country. Rise in sea levels has been experienced, with Islands of Maziwe and Fungu la Nyani been submerged due to rise in sea level. Among others, this paper describes climate change in Tanzania in detail and its impacts to natural ecosystems.R evolution of computers in last decades provides complicating atmospheric models with detailing vertical profiles, accounting for irregular clouds in wide spectral ranges. Numerical algorithms for calculating radiative characteristics with maximal exactness and minimization of uncertainty are usually applied. There are many different computer codes including look-up tables with aerosols characteristics, water vapor, atmosphere conditions in different latitudes, continental and sea conditions, and seasons. Sophisticated approaches for calculating optical parameters are based on scattering and radiative transfer theories. It is very useful for applied problems. However the analysis of separate factors influence on atmospheric radiative characteristics without of considering all possible variations of the whole totality is often necessary for many research problems. For that case the simple models of homogeneous (for the clear atmosphere) and two or three layer atmosphere (for cloudy cases) allow operative varying considered atmospheric optical parameters and provide result that hardly contributes to complicate models and clearly elucidateaninteractions between of key atmospheric parameters and radiative characteristics. Two-stream methods of radiative transfer theory ensure an acceptable exactness for calculating integral (over viewing directions) radiative characteristics (irradiance and radiative divergences). Asymptotic formulas are also effective for fast and transparent calculation in case of the cloud atmosphere. A simplest optical model is accepted of the homogeneous clear atmosphere including ozone absorption in UV ranges, molecular scattering, and four variants of the aerosol content at selected shortwave wavelength. In cloud case three variants of extended cloud layer are added. Radiative characteristics together with heating rate are calculated and presented in this study. Results of optical parameters retrieved from observation of solar radiation in the atmosphere and radiative characteristics are compared with simple modelling.